In the known methods of quantitative analysis, with one measurement, only the quantity of a single component in a given volume is determined, such as the mass density or numerical particle density of the respective substance. If, however, there is no need for knowing the quantity of the substance but only the proportion thereof relative to the quantity of another component present in the given volume, the measurement either must be carried out under well-defined conditions (for example, control the temperature or pressure, or removing accompanying substances) or must be complemented by another measurement.
Determining the concentration ratio of two components of a mixture of substances is a problem which is posed very often. That is, in almost all chemical reactions, it matters to observe definite concentration ratios, in the chemical technology, not least for reasons of economy. But also in many other fields, the concentration ratio is determinative, sometimes even in cases where it is not manifest at first sight, of which examples will be given hereinafter.